One of the most impacted cities was Houston, where rainfall levels totaled 43.38 inches. In Cedar Bayou, Texas — about 30 miles east from the Houston’s downtown — a rain gauge measured 51.88 inches of rainfall, breaking the record for the continental United States set in 1978.

Flood experts say that ust two main factors will rid Houston and its surrounding counties of all the water: gravity and time.

“You can’t drain it any faster than the system allows, and that’s being driven by gravity, the capacity of channels and the overbank areas, and the floodplains to convey that water into the Houston shipping channel and ultimately into the Gulf of Mexico,” Kevin Stewart, a flood warning services manager at the Urban Drainage and Flood Control District (UDFCD) in Colorado, told Business Insider.

Since rain stopped falling, Harvey’s water has been rushing toward and into the Gulf of Mexico, which will be its main way out. Houston’s shipping channel is funelling water into the Gulf, but not that quickly, Stewart said. Texas water management officials have also been balancing flood levels using reservoirs and dams.

“Houston is familiar with getting eight to 10 inches of rain. It’s not an unusual thing for the area to see those kinds of rainfalls. But to see 40 to 50 inches, that’s just catastrophic,” Stewart said.

In West Houston, people are being asked to evacuate, due to the potential for more flooding — not from more rainfall, but because build up from the Addicks and Barker reservoirs need to flush through the area to move floodwater closer to the Gulf. Rain from Harvey has filled the reservoirs to capacity, USA Today reported.

Since much of the soil in Houston contains clay — which doesn’t soak up water well — some Harvey water will also evaporate, Daniel Hitchcock, a Clemson University professor who researches flood management, told Business Insider.

He added that building smaller-scale green infrastructure — like rain gardens (i.e basins that absorb flood water ) and bio-retention cells (i.e. grassy areas that store flood water and runoff) — can help mitigate future flooding in the region. Harris County could widen existing drainage systems or buil more levees to mitigate flooding as well.

“The more we build, the more dense developments get, and the more that we pave over, the natural drainage systems — the soil — aren’t going to work, and the more runoff we’re going to have,” Hitchcock said, noting that Harvey was a rare flood. “Clearly, in the urban areas, there are significant challenges for how to manage all that water. With Harvey, it was so much water, that I’m not sure what you can design for something like that.”

Water damage recovery will likely take months in the greater Houston area, Stewart estimates. In an interview with The Washington Post, Richard Long, a natural resource management specialist with the Army Corps of Engineers, reiterated that timeline.

“We don’t know what Mother Nature’s going to give us,” Long said last Tuesday. And either way, he added, “it’s going to take quite a while for us to get rid of all this water.”

Later this week, the northern Caribbean, Puerto Rico, the US Gulf Coast, as well as East Coast states from Florida and Maine could see more flooding with Hurricane Irma, a storm that has now reached Category 5 strength with sustained winds of 175 mph.

Though Irma’s path is still speculative, officials say cities anywhere near the eye of the storm should prepare for dangerous rainfall and wind speeds.